The present invention relates to a supply voltage decoupling device for HF amplifier circuits, comprising an output line for the output of an amplified signal, wherein one end of the output line, which is not used for signal output, is connected to a circuit element designed as decoupling circuit.
The device is provided in particular for very broad band distributed amplifiers, such as those employed in the optical communication technology. For the utilisation of its high potential in terms of transmission bandwidth, the optical communication technology requires electronic amplifier circuits capable of coping even with waves in the millimeter range while starting out from frequencies in the audible range. This requires a bandwidth in amplification that ranges from a few kHz up to a few tens of GHz.
For the operation of the employed amplifier circuits decoupling of the DC voltage from the signal-carrying part of the circuit is necessary, specifically in the manner provided by the present inventive device.
Many amplifier circuits for optical communication technology are presently designed in a monolithic structure. In amplifiers of such a type the supply voltage is decoupled from the amplifiers via decoupling circuits composed of capacitors, coils and resistors. There, the resistor is connected in series with the coil, as may be seen, for example, from the illustration in the data sheet xe2x80x9cMonolithic Amplifiersxe2x80x9d of the company of Mini-Circuits(copyright). In the case of very low amplifier outputs this is generally not problematic because the losses occurring on the resistor, due to the direct voltage component of the supply voltage, create only small thermal outputs that can be easily carried off without major expenditure.
However, this concept cannot be employed in the case of higher power input levels, particularly when integrated amplifiers are used that are designed for higher output levels, because in such a case a substantial additional thermal output is produced in a minimum of space and must be carried off under the aggravated spatial situation. The application of a major number of such components in respective installations moreover entails a noticeable additional expenditure at the system level.
As a rule, specific separate decoupling circuits in a hybrid circuitry design are used in the case of amplifiers for higher power demands, which are composed of discrete inductors and capacitors. Such separate decoupler circuits are, however, very expensive and require a space much greater than that common for integrated systems. One example of such a decoupling circuit can be seen in the data sheet of SHFdesign(copyright), which relates to the unit xe2x80x9cWideband Bias-T SHF 122xe2x80x9d. The precise details of the circuitry in such systems are not known, however. Apart from the substantial dimensions, such decoupling circuits must be directly connected in the signal path so that additional attenuation and distortion of the signal must be expected.
Moreover, the following circuits are known in relation to the supply of supply voltage to amplifiers.
The German Patent DE 3117009 discloses a filter network constituted by two inductors as well as two capacitors. This filter network does not allow the flow of a high-frequency current from the drain electrode of the transistor to the source of supply voltage. In the case of application in a distributed amplifierxe2x80x94rather than the inventive devicexe2x80x94the filter network would not absorb high-frequency currents without reflection but rather reflect these currents, thus impairing the output reflection factor of the amplifier as well as the flat development of amplification towards values that are not useful.
The German Patent DE 19534382 presents a method employed for amplifiers having a bandwidth other than an ultra-wide bandwidth for supplying supply voltages by means of high-frequency lines having the length of a quarter of the wavelength approximately at the operating frequency. One special feature in this case is the reduction of the necessary length of the line by means of an inductor. This method does, as a matter of fact, not operate properly in the case of a distributed amplifier having an ultra-wide bandwidth, because the substitution of the inventive device by this method results, in its turn, in undesirable reflection. Even the operation in parallel with a suitable terminating resistor results in a restriction of he useful frequency range in a direction away from the very low frequencies that are equally relevant in terms of transmission technology.
The German Patent DE 19752216 discloses circuits for the DC power supply in amplifiers, which are essentially composed of inductors, resistors as well as capacitors. In these circuits a DC current flows through the resistor R2, which, in the case of a power amplifier, gives rise to inexpedient effects with respect to the efficiency and the heat produced. If the value of R2 disappears a low-pass filter is formed by C2, C4 as well as Ls2, whose applicationxe2x80x94instead of the inventive devicexe2x80x94gives rise to the same negative effects as those described already with reference to the German Patent DE 19534382.
The U.S. Pat. No. 5,349,306 discloses the supply of DC current into the drain line of a distributed amplifier, with the drain voltage Vdd being passed via a capacitor through a coil that is part of a band-pass network combining the drains of the FETs. In this manner, a point in the band-pass network, which is cold in terms of high frequencies, is skilfully utilised for coupling in a DC current. However, a band-pass requires a high lower cutoff frequency. The prerequisite for a distributed amplifier with the lowermost lower cutoff frequency, by contrast, is the application of a low-pass filter network that does not involve this possibility of DC voltage supply.
It is an object of the present invention to provide a device for supply voltage decoupling for HF amplifier circuits, which is suitable for elevated amplification outputs and which can be implemented at low costs. The device should particularly permit the power supply of integrated distributed amplifiers designed for elevated outputs at a low power loss caused by the decoupling circuit.
This object is achieved with a device of the type defined in patent claims 1, 6, 7, 8 and 9. Expedient embodiments of the device are subject of the dependent claims.
The devices comprise an output line connected to the HF amplifiers for coupling out an amplified signal, wherein one end of the output line, which is not used for coupling out the signals, is connected to a circuit element designed as a decoupling circuit. In distinction from the known decoupling circuits according to prior art, the present circuit element does not present any non parasitic ohmic resistance for DC power. The parasitic ohmic resistance of the circuit element is therefore smallxe2x80x94compared against the line impedance of the output line so that, with a line impedance of 50 xcexa9, it will not essentially exceed a value of 5 xcexa9 in the most inexpedient case. The avoidance of a parasitic resistance is preferably the aim. The circuit element is moreover designed in such a way that it produces an HF power absorption increasing as the frequency increases, and constitutes a reflection-free load for high frequencies.
The supply voltage is supplied to the HF amplifier or amplifiers via this circuit element. Due to the low ohmic resistance of the circuit element the power loss due to the applied supply voltage is only small. The demands in terms of provisions for carrying off the heat can therefore still be managed without any problems, even in the case of elevated power levels. The absorption of the circuit element, which is reduced as the frequency increases, as well as its function as reflection-free high-frequency load permit the trouble-free coupling-out of the amplified signal at the other end of the output line.
In a preferred and very expedient embodiment of the present device, the circuit element consists of several subcircuits operated in a cascade, whereof each consists of an inductor on a first part of the line as well as of a capacitor and a terminating resistor on a second line segment. The second line segment here branches off the first line segment and is connected to the ground whilst the first line segment of a subcircuit is respectively connected to the first line segment of the subcircuit joining it in the cascade. In this manner, a series circuit of inductors is created between which the respective second line segment with the capacitor and the terminating resistor is branched off.
This catenary arrangement of subcircuits as the present circuit element is connected to that end of the output line of the preferably distributed amplifier, which is not used for carrying off the amplified signal, instead of the terminating resistor required in usual designs. In a particularly expedient improvement of the device, the power outputs of higher frequencies are isolated and absorbed by a first number of subcircuits operated in a cascade whilst the remaining power outputs at comparatively low frequencies are absorbed by similar circuits that are, however, joined to the chip, i.e. not integrated in the chip, and are differently dimensioned in an appropriate manner.
One essential advantage of the present device mainly consists in the fact that the generation of thermal power by means of the d.c. current to be supplied to the amplifiers is avoided. As on that end of the output line of the preferably distributed amplifier, which is not used to carry off the amplified signal, a substantial fraction of the signal power appears only at comparatively low frequencies it is possible that the first elements of the catenary arrangement of subcircuits, which isolate signal components of higher frequencies, are implemented on the amplifier chip because in such a case they produce only a small power loss. Higher power losses are created in the subcircuits joined to the chip, which, due to the small residual bandwidth of the signal to be decoupled from the supply current, can be implemented at a comparatively low expenditure and hence at low costs. On the other hand, the first subcircuits for the absorption of the highest frequency components are integrated into the amplifier chip and can hence be equally implemented at low costs.
In this case the first subcircuits are designed to present a lower absolute value of the induction and hence a higher cutoff frequency as well as a higher self-resonant frequency of the inductor than the following subcircuits. The inductance preferably increases and the cutoff frequency of each subcircuit as well as the self-resonant frequency of the inductance of each subcircuit decreases as the number of subcircuits connected already to the output line increases.
In addition to the preferred embodiment with individual discrete subcircuits, the present circuit element may also be implemented in a different manner. The entirety of all subdevices may be considered to be a dissipative catenary conductor (ladder network). Its infinitesimal correspondence is a dissipative high-frequency line that may be equally used as circuit element at the end of the output line with the characteristics described here. Moreover, dissipative lines involving an insulating attenuating material may be implemented and employed with the specified characteristics. Further examples of circuit elements that may be designed in correspondence with the specified requirements are micro-strip transmission lines with an attached absorber wedge made of ferrite or with a ferrite paste applied thereon. Such systems are also known by the term xe2x80x9cmicro-wave marshxe2x80x9d.
In another embodiment, the circuit element includes optional low-pass filters matched with the effective impedance of the output line or whose effects produce a corrective influence on the amplifier characteristics in transmission.